Steps to reproduce the issue

1. Create a subform field with a calendar, see example code below
2. Load the page, the following alert appears:
   

subform.xml

<?xml version="1.0" encoding="UTF-8"?>
<form>
		<field
				name="date"
				type="calendar"
				label="JDATE"
				format="%d-%m-%Y"
				filter="user_utc"
		/>
</form>

Joomla! field (assuming we use mod_custom)

<field
		name="my-field"
                type="subform"
                formsource="modules/mod_custom/forms/subform.xml"
                multiple="true"
                min="1"
                layout="joomla.form.field.subform.repeatable-table"
                label="my subform field"
/>

Expected result

The calendar field is load after the subform field. So the calendar is working.

Actual result

The Calendar setup is load before the subformfield, so the alert that the field can't be found is triggered. When click on the "oke" button the subform is load and the calendar field is working.

System information (as much as possible)

Joomla! 3.6.4
Latest version of Google Chrome

Additional comments

Why do we even trigger a alert? Why not just a simple console.log?